Microlensing of Quasars by Stars in Their Damped Lyα Absorbers

The damped Lyman alpha absorbers (DLAs) in quasar spectra are believed to bethe progenitors of present-day disk galaxies. We examine the probability formicrolensing of background quasars by stars in their DLAs. Microlensing by anindividual star should magnify the continuum but not the broad emission linesof the quasars. Consequently, the equivalent width distribution of microlensedquasars would be distorted. We model a representative spiral galaxy as a closedsystem composed of a bulge, a disk, and a halo, and evolve the mass fraction ofstars in the disk based on the observed metallicity of DLAs at high redshifts.The microlensing signatures are stronger if the halo of the galaxy is made ofMassive Compact Halo Objects (MACHOs). In this case, the distortion imprintedby microlensing on the equivalent width distribution of quasar emission linescan be detected with high significance in a sample of about 10 DLAs with HIcolumn densities N>10^21 cm^-2 and absorption redshifts z_abs<1. About a tenthof all quasars with DLAs (N>10^20 cm^-2) might show excess variability ontimescales shorter than five years. A search for these signals would complementmicrolensing searches in local galaxies and calibrate the MACHO mass fractionin galactic halos at high redshifts.Comment: 25 pages, 8 figures, final version accepted for publication in Ap